Noise can present an irritating, or even dangerous, problem in a variety of environments. For instance, noise in residences and commercial buildings can be irritating, whereas noise generated by aircraft engines and in certain industrial environments, for example, may even harm individuals inside the aircraft or proximate to the industrial equipment generating the noise. Noise from aircraft or industrial sites can also disturb or harm nearby communities.
In the context of aircraft engines, broadband acoustic absorbers are beneficial for reducing noise produced by aircraft engines. This may be an even more acute problem for aircraft engines with short inlet ducts, convoluted inlet ducts, or obstructed inlet ducts. Engines with short, convoluted, or obstructed inlet ducts may produce noise, possibly at frequencies below 1000 Hz, due to disturbances in the flow entering the engine.
To reduce the noise propagating from aircraft engines, a combination bulk absorber-honeycomb acoustic panel designed for the duct of an aircraft turbofan engine has previously been described in U.S. Pat. No. 4,235,303. This design alleges to use a perforate over honeycomb absorber coupled to a broadband noise suppressing bulk absorber material that can be generically described as a finely divided felted or woven material, either organic or inorganic, having a high space-to-solid material ratio. Suitable bulk absorber materials are listed as porous ceramics, goose down, steel wool, Kevlar, and Scotfelt™. Preferably, the bulk absorber material is capable of attenuating noise in the range of 50 to 10,000 Hz.
A number of challenges are described in U.S. Pat. No. 4,235,303. Primarily, the bulk absorber and honeycomb need to be protected from water, oil, and dirt contamination while simultaneously exposed to the incident sound wave. U.S. Pat. No. 4,235,303 allegedly overcomes these challenges by sandwiching the bulk absorber and a honeycomb within layers of perforate, which allows liquid contaminants to drain in an effort to protect the acoustic panel and maintain adequate acoustic performance of the bulk absorber and honeycomb.
Another problem with bulk absorbers is that as the layers become thinner, as in an attempt to minimize weight and volume required for installation, the acoustic absorption coefficient decreases particularly at the lower frequencies, such that it generally remains a challenge to absorb sound with thin lightweight bulk absorbers at frequencies below 1,000 Hz. The honeycomb panels of U.S. Pat. No. 4,235,303, with large open pores arranged perpendicular to the direction of airflow through the engine, are typically used to reduce sound for a narrow range of frequencies, and that frequency range is dependent upon the depth of the channel.
It generally remains a challenge to absorb sound below 1,000 Hz with thin, lightweight honeycomb-like materials as well, given the typical space constraints and the requirement to survive in the harsh operating environment of an aircraft engine, and indeed in the 400-3,000 Hz frequency range generally. Accordingly, improved acoustic absorbers may be beneficial.